1. Field of the Invention
The present invention relates to an apparatus for detecting the state of a heat exchanger tube, and more particularly, to an apparatus for concurrently detecting a defect generated in a heat exchanger tube and a geometric transition of the heat exchanger tube.
2. Description of the Related Art
The general terminology, heat exchanger tube, includes steam generator tubes and condenser tubes of nuclear power plants, boiler tubes in fossil fuel power plants, and the like. There may be a defect in a heat exchanger tube, such as a crack, a wear, a pit, and an intergranular corrosion. In order to detect a defect of a heat exchanger tube, there is an eddy current test as a conventional nondestructive testing method.
Current conventional eddy current tests include using a probe equipped with a non-surface riding coil and using a probe equipped with a surface riding coil.
Conventional non-surface riding coil probes include a bobbin coil probe and a multi-array coil probe in which a coil is separated from the inner surface of a heat exchanger tube. However, in case of a conventional non-surface riding coil probe, a geometric transition of the inner/outer surface of a heat exchanger tube generates an eddy current signal caused by a gap change between the inner surface of the heat exchanger tube and the coil. In this case, if a defect exists in a shape-changed position, since an eddy current signal caused by a defect is not distinguished from an eddy current signal caused by a geometric transition, a technical weak point of this method is the difficulty in detecting a defect.
To solve the problem, a rotating pancake coil probe is developed, which is a surface riding type probe equipped with springs below a coil such that the coil is in constant contact with the inner surface of a heat exchanger tube. The rotating pancake coil probe rotates the probe by using a motor while moving up and down the heat exchanger tube in a straight line to test a heat exchanger tube. However, in the conventional rotating pancake coil probe, since a coil is in constant contact with the inner surface, in the case a geometric transition exists on the inner/outer surfaces of the heat exchanger tube, an eddy current signal caused by a gap may be minimized, thereby measuring only the eddy current signal caused by a defect, regardless of the geometric transition.
As describe above, due to the characteristics of each conventional probe, in a eddy current test of a commercial power plant, a procedure of accurately detecting a defect by using a surface riding type rotating pancake coil probe on a region of a heat exchanger tube, in which an abnormal signal is generated, after performing a test by using a non-surface riding type probe such as bobbin coil probe is applied. Accordingly, it is impossible to concurrently detect a defect developed in the heat exchanger tube and a geometric transition around the defect, and also to distinguish the defect from the geometric transition by using the conventional testing method.
On the other hand, there may be many types of geometric transitions on the inner/outer surfaces of a heat exchanger tube, such as an expansion, an expansion transition, a bend, a dent in which a local zone is intruded in the direction of the inside diameter, and a bulge in which a local zone is protruded in the direction of the outside diameter. Korean Patent Application No. 10-2004-0006391 discloses an eddy current probe for measuring only a geometric transition in a heat exchanger tube, and there is a technical restriction of not being able to detect a defect in a shape-changed position.
Since the described geometric transition causes a local residual stress in a heat exchanger tube and acts as a structural factor accelerating the evolution of a defect, in particular a crack type defect, it is required to develop a method of concurrently detecting a geometric transition in addition to a defect and distinguishing the defect from the geometric transition in an eddy current test.